"I do cardio. I run. I strength-train using my own body weight," the first American Idol winner tells Redbook for its July issue. "I don't like free weights, because I build muscle easily."

Clarkson, 30, points out that shaping up is all about finding what's right for you. "Really, what I try to instill in my fans is to be healthy and happy," she says. "I have no desire to be super-skinny."

As for imparting tips to up-and-coming singers on her show, she says she's giving the Duets contestants something she never had on Idol.

"I'm going to be completely honest – I had no one looking out for me. They were looking out for the show, and it was horrible," she says. "I was 19 years old. I was smart enough to know that it wasn't about me, but that didn't make it any easier. I'm all cried out about it, but my first four years, I hated my life; I almost quit a dozen times."

Now, thanks in part to her boyfriend Brandon Blackstock, she's happy. "Where's the kick in the ass? I told my assistant that today," she says. "I said, 'I'm just waiting for something to blow up.' She said, 'No, that's what happiness is.' I'm like, maybe I've never been happy! Because Brandon does – he makes me happy."